Ability to store selections
Sadly this feature will most likely never make it into the official release of blender. 

As CoDEmanX said, 


  It's not possible to implement it well in Python, as custom layer data propagates on actions like extrusion. So if you stored the selection state of edges or faces as a property of these elements, it would be copied over to the new elements when you extrude etc. 


Also it is important to notice that there are some selection difficulties that would come into play. in spite of all that, there is a proposal for such a feature, and it may become possible in the future.